Semiconductor memories provide memory storage for electronic devices and have become very popular in the electronic products industry. In general, many semiconductor chips are typically fabricated (or built) on a silicon wafer. The semiconductor chips are individually separated from the wafer for subsequent use as memory in electronic devices. In this regard, the semiconductor chips contain an array of memory cells that are configured to store retrievable data, often characterized by the logic values of 0 and 1.
One class of semiconductor memories is resistive memories. They typically use two or more different resistive values of a switchable resistor to define cell states in the memory useful in storing data. One particular type of resistive memory is a phase change memory. In one known structure of a phase change memory cell, the memory cell is formed at the intersection of a phase change memory material and an electrode. Passing energy of an appropriate value through the electrode heats the phase change memory cell, thus affecting a phase/state change in its atomic structure. The phase change memory cell can be selectively switched between logic states 0 and 1, for example, and/or selectively switched between multiple logic states.
Materials that exhibit the above-noted phase change memory characteristics include elements of Group VI of the periodic table (such as Tellurium and Selenium) and their alloys, referred to as chalcogenides or chalcogenic materials. Other non-chalcogenide materials also exhibit phase change memory characteristics.
The atomic structure of one type of phase change memory cell can be switched between an amorphous state and one or more crystalline states. The amorphous state has greater electrical resistance than the crystalline state(s), and typically includes a disordered atomic structure with only short range coordination. In contrast, the crystalline states each generally have a highly ordered atomic structure, and the more ordered the atomic structure of the crystalline state, the lower the electrical resistance (and the higher the electrical conductivity).
The atomic structure of a phase change material becomes highly ordered when maintained at (or slightly above) the crystallization temperature. A subsequent slow cooling of the material results in a stable orientation of the atomic structure in the highly ordered (crystalline) state. To switch back, or reset to the amorphous state, for example in the chalcogenide material, the local temperature is generally raised above the melting temperature (approximately 600 degrees Celsius) to achieve a highly random atomic structure, and then rapidly cooled to “lock” the atomic structure in the amorphous state.
The temperature-induced set/rest changes in memory states may be achieved in a variety of ways. For example, a laser can be directed to the phase change material, current can be driven through the phase change material, or current can be passed through a resistive heater adjacent the phase change material. In any of these methods, controlled heating of the phase change material causes controlled phase change within the phase change material.
The temperature-induced set/rest changes in the memory cell(s) create locally elevated temperatures, or hot spots, within each cell. Ineffective thermal isolation of hot spots in memory cells requires an increase in current (and thus power) to reset a memory state in the memory cell. It is desired to reduce the power needed to change memory states in memory cells to enable the use of smaller selection devices, thus reducing an overall size for memory devices, in general.
For these and other reasons, there is a need for the present invention.